PowerPedia:BetaVoltaic

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For a list of resources on betavoltaics that harness electrons given off in radioactive decay, see Directory:BetaVoltaics

Betavoltaics is an alternative energy technology that promises vastly extended battery life and power density over current technologies. Betavoltaics are generators of electrical current, in effect a form of battery, which use energy from a radioactive source emitting beta particles (electrons). The functioning of a betavoltaic device is somewhat similar to a solar panel, which converts photons (light) into electric current. This type of radioactive battery (or nuclear battery) operate on the continuous radioactive decay of certain elements. These theoretical batteries last a long time.

Contents

History

Betavoltaics were invented over 50 years ago. Betavoltaic power cells are sometimes referred to as betavoltaic batteries, atomic batteries, nuclear batteries, nuclear micro-power sources / devices, or stimulated / accelerated isotope decay power cells. They are sometimes described with the prefix "long-lived" since theoretically they can last as much as 20 years or more.

They have been developed since the 1950's. They were initially desgined to meet the high-voltage, high-current draw requirements of electrically powered space probes and satellites. (For example, the Army Research Lab did betavoltaic testing in 1954 using dissimilar metals. In one device not much larger than a car battery they were successful in achieving a 70 watt output for a short time). As early as 1973, betavoltaics were suggested for use in long-term medical devices such as pacemakers.

The modern Betavoltaic power cell's standard operating voltage is between 100kV and 1.5 million kV in potential, however they are capable of being adapted for lower voltage power requirements. In 2005 a new betavoltaic device using porous silicon diodes was proposed to increase their efficiency. This increase in efficiency is largely due to the larger surface area of the capture material. The porous silicon allows the tritium gas to penetrate into many pits and pores, greatly increasing the effective surface area of the device.

Betavoltaic operation

A betavoltaic power cell is composed of semiconductors and at least mildly radioactive material.As the radioactive isotope decays, it emits beta particles (electrons). Betavoltaic devices are not "free energy" or over-unity devices. Defintions to remember in discussion of thier operation include:

Beta: meaning beta-electron, highly energetic electrons / positrons ejected during the decay of a neutron into a proton.

Betavoltaic power cell / battery: a device that captures beta-electrons emitted by a decaying radio-isotope for the purpose of producing useable electric power.

In a betavoltaic, when an electron strikes a particular interface between two layers of material (a p-n junction), a current is generated. Whether betavoltaics will replace current battery technologies altogether remains to be seen. Recent developments however, are promising. The following is meant to provide a basic introduction to betavoltaics in general and the current state of the art in betavoltaic technology. A common source used in betavoltaicsis the hydrogen isotope, tritium. Unlike most nuclear power sources, which use nuclear radiation to generate heat, which then generates electricity (thermoelectric and thermionic sources), betavoltaics use a non-thermal conversion process.

Betavoltaic devices use radioactive isotopes as their source of fuel, that is why they are sometimes called radioactive batteries. Betavoltaic devices are not nuclear reactors in the traditional sense. Unlike typical nuclear power generating devices, betavoltaic power cells do not rely on a nuclear reaction (fission / fusion) or chemical processes (as in most batteries) and do not produce radioactive waste products. The atomic nuclei (protons and neutrons,) is not split apart or fused with other nuclei. Rather, this process takes advantage of beta (electron) emissions that occur when a neutron decays into a proton. Internally, the impact of the beta electron on the P/N junction material causes a forward bias in the semiconductor. This makes the betavoltaic cell a forward bias diode of sorts, similar in some respects to a photovoltaic (solar) cell. Electrons scatter out of their normal orbits in the semiconductor and into the circuit creating a useable electric current. (Ed. a simple simple-beta gif image is availble.)

Although betavoltaics use a radioactive material as a power source, it is important to note that beta particles are low energy and easily stopped by shielding, as compared to the gamma rays generated by more dangerous radioactive materials. With proper device construction (i.e.: shielding), a betavoltaic device would not emit any dangerous radiation. Leakage of the enclosed material would of course engender health risks, just as leakage of the materials in other types of batteries lead to significant health and environmental concerns.

The theory behind betavoltaic devices are relativistic in nature: protons and neutrons are essentially highly compressed and more or less stable forms of energy (E=MC2). The decay of a neutron into a proton releases large amounts of electrical energy. Neutron beta-decay into protons is said to be the world's most concentrated source of electricity.

Limitations

Several limitations can inhibit the efficiency of betavoltaic power cells. One limitation of betavoltaic power cells is the re-absorption of electrons in the radioactive source itself. In order to reduce the self-absorption of beta energy, the radioactive isotope must be incorporated into the lattice of a semiconductor.

Another limitation is that the highly energetic electrons tend to wear down or break apart the internal components (semiconductors) of the power cell. Betavoltaic devices suffer internal damage to their components as a result of the energetic electrons. Additionally, as the radioactive material emits, it slowly decreases in activity (refer to half-life). Thus, over time a betavoltaic device will output less and less power. This decrease occurs over a period of many years.

In device design, one must account for what battery characteristics are required at end-of-life, and insure that the beginning-of-life properties take into account the desired useable lifetime. Much of the betavoltaic research conducted over the years has been in identifying more durable semiconductors for power cell applications. Materials research in long lasting semiconductors which can take the punishment of long term exposure to beta-electron impact has been conducted by Sandia National Labs of NM in partnership with the University of New Mexico. One promising material is called Icosahedral Boride. This is a very hard semiconductor material which may lead to a technology for direct conversion of the beta electron energy into electric current. The unique structure of these semiconductors allows for the design of safe, high-output devices.

Environmental impact

The isotopes used in stimulated decay betavoltaic devices are electronically pumped. They are inert when the cell runs out of power. This eliminates the possibility of toxic or radioactive waste. A thick epoxy shield prevents the chemical components of these cells from leaking out and entering the environment. Also potentially useful in sheilding these devices are a new class of metals called "liquid metals": metals with amorphous (as opposed to crystalline) structures. The long life-span of these devices (as much as 10 to 30 years of continuous use,) means that many fewer of them need to be sold than regular batteries thus helping to minimize environmental impact.

New approaches

Tritium Batteries

Thin-film tritiated amorphous silicon cells have been built. These cells are often called tritium batteries. Tritium batteries are cheap, long-life, high energy density, low-power batteries. They have a specific power of 24 watts per kilogram, a full load operating life of 10 years, and an overall efficiency on the order of 25%. Tritium is readily substituted for the hydrogen present in hydrogenated amorphous semiconductors. Tritiated amophous films are mechanically stable, free from flaking or blistering, with good adherence to the substrate and may be simultaneously deposited onto both conducting and insulating substrates. The deposition technique is a discharge in tritium plasma. The silicon layer sputtered in a tritium / argon ambient environment at temperatures below 300'C results in a tritiated amorphous silicon film with the tritium concentration varying from 5 to 30% depending upon deposition conditions. Tritium however, is typically only produced inside traditional nuclear (fission) reactors. Radioisotopes other than tritium, may also be used as a source of energetic electrons such as krypton-85 for example.

Stimulated Decay Batteries

One new development in betavoltaic technology are the attempts at controlling output by artificially stimulating / accelerating the natural beta-decay rates of various materials. This stimulation is usually electro-magnetic and / or acoustic in nature. A common misconception is that electro-magnetic stimulation of atomic nuclei is impossible given that the nucleus is highly resistant to electron bombardment. Electrons are typically repelled by the nucleus. Rather than enter the nucleus, they instead tend to orbit in cloud-like formations. When electrons are expelled from the nucleus during beta decay, they are expelled violently with extremely high velocities, (hence the term "high energy electron" or "beta-particle".) However, the methods typically used in beta decay stimulation / acceleration involve so-called "standing wave" technology (also called "longitudinal" or "scalar waves". See also "scalar interferometry".)

Some promising fuel sources for stimulated beta-decay include:

Potassium-40

Molybdenum-100

Zinc-70

These isotopes have a decay rate of many thousands of years. For this reason they are not regulated by the United States government as are many more energetically radioactive materials (those with short half-lives, that is: rapid decay rates). These isotopes have been found to have significent beta decay energy. Many of these are light metals and can be inexpensively plated.

Applications

The primary use for betavoltaics is for remote and long-term use, such as application requiring electrical power for a decade or two. The recent progress in technology has prompted some to suggest using betavoltaics to trickle-charge conventional batteries in consumer devices, such as cell phones and laptop computers. Betavoltaic applications include:

Aerospace: satellite and other unmanned vehicle power supplies.

Power industry: power sources, backup power sources, and remediation of radioactive waste through artificial acceleration of natural isotopic decay rates.

Although consumer applications are being developed, it is uncertain whether consumers will be willing to adopt "personal nuclear technology" given the pervasive negative sentiment toward nuclear power in general as inherently unsafe.

Summary

Betavoltaic technology is the science of deriving useful electrical power from the beta decay of certain radioactive isotopes. There are inherent theoretical limits to the efficiency and output of betavoltaic devices, however their output even at low efficiencies can be quite significant. Betavoltaic technology has a fairly long history (50 years or more,) but has benefitted significantly from recent breakthroughs in materials science, nanotechnology and quantum electrodynamics. Betavoltaic technology holds particular promise for the aerospace, security, and power industries. The environmental impact of this technology appears minimal especially when compared with current battery technology.